I don't leave height out of the equation. Thomas is exactly as effective as Lillard even though he is 6 inches shorter.
Lillard's height has not helped him to shoot a better career%. Lillard's height has not help him to be a better defender (by every conventional and advanced metric). Lillard's height has not helped him to score more effectively or efficiently.
I admit that Lillard can score in more ways than Thomas, but Thomas is more effective than Lillard at the ways he can score, meaning that they even out.
While height is a big thing, some players defy conventional logic when it comes to height. They find the ability to be just as effective (if not more so) as a taller player. Think of Charles Barkley, Ben Wallace, Allen Iverson, Muggsy Boggues, Elton Brand, Paul Milsap, Eric Gordon, Kenneth Faried, Draymond Green, Monta Ellis, Tony Allen, etc. Each one of these players played different ways with different strengths, but by conventional wisdom, they should not have been able to be effective because of their height.
This is the category Thomas is in. Like it or not, Celtic fans will need to embrace that CBS does not care about height. He cares about speed, skill, and effort. This means 4 of the 5 positions in our starting lineup could be undersized by conventional thinking (Thomas, Smart, Crowder, Sullinger), but as long as they are effective, who cares?